To remove carbon-based particulate matter from exhaust gases emitted from diesel engines, a ceramic honeycomb filter comprising a ceramic honeycomb structure with both ends alternately sealed has been used. As shown in FIGS. 3(a) and 3(b), a ceramic honeycomb structure 20 comprises a ceramic honeycomb structure 10 comprising porous cell walls 2 which form a lot of flow paths 3 and an outer peripheral wall 1; and plugs 4a, 4b alternately sealing both end surfaces 5a, 5b of the flow paths 3 in a checkerboard pattern. An exhaust gas G containing particulate matter enters the flow paths 3a that are open at the inlet end 5a, passes through the cell walls 2, and exits from the flow paths 3b that are open at the outlet end surface 5b via adjacent flow paths 3b. During this process, particulate matter in the exhaust gas G is captured by pores (not shown) in the cell walls 2.
The formation of plugs in both end portions of a ceramic honeycomb structure has conventionally been conducted, for example, by the method shown in FIGS. 4(a) to 4(e). A resin film 6 is attached to an end surface 5a of the ceramic honeycomb structure 10 as shown in FIG. 4(a), and provided with holes 6a by laser beam such that the flow paths of the ceramic honeycomb structure are arranged in a checkerboard pattern as shown in FIG. 4(b). Thereafter, the ceramic honeycomb structure 10 is immersed in a plugging material slurry 8 comprising ceramic powder and a dispersing medium in a container 40 as shown in FIG. 4(c). During immersion, the ceramic honeycomb structure 10 is pushed downward (shown by the arrow D). The plugging material slurry 8 enters the predetermined flow paths 3 through the holes 6a in the sealing film 6 to form plugs 4a. After the ceramic honeycomb structure 10 is taken out of the container 40. plugs 4b are similarly formed in the other flow paths from the other end 5b. The plugs 4a, 4b are then sintered, so that the end surfaces 5a, 5b of the ceramic honeycomb filter 20 are sealed in a checkerboard pattern.
In the conventional method shown in FIG. 4(e), however, recesses 7 are likely generated at end surfaces 45a and 45b of the plugs 4a, 4b. FIGS. 5(a) to 5(d) show a plug 4 in a flow path 3 in an enlarged manner. FIG. 5(a) shows a normal plug 4, and FIGS. 5(b) to 5(c) show a plug 4 having a recess 7 generated in its end 45. In the extreme case shown in FIG. 5(c), the recess 7 is a through-hole 7a, through which particulate matter in an exhaust gas leaks. Although the penetration of the recess 7 can be avoided by elongating the plug from 4 to 4d′ as shown in FIG. 5(d), the elongation of the plug 4 undesirably makes the flow path 3 (corresponding to the effective area of the cell wall 2 for capturing particulate matter in an exhaust gas) shorter.
To avoid the generation of recesses during the formation of plugs, as shown in FIGS. 6(a) and 6(b), JP 2004-25098A discloses a method comprising pressing a plugging material slurry 8 in a container 40 into flow paths 3 of a ceramic honeycomb structure 10 to form plugs 4; sliding a bottom plate 41 of the container 40 to open the bottom of the container 40; and taking the ceramic honeycomb structure 10 out of the container 40 by lifting. JP 2004-25098 A discloses that if the bottom of the container 40 is opened, negative pressure is not applied to the plugs 4 when the ceramic honeycomb structure 10 is lifted, so that the generation of the recesses 7 can be avoided. As a result of experiment, however, the inventors have found that in the method of JP 2004-25098 A, contact resistance would be too high to slide the bottom plate of the container 40 if the ceramic honeycomb structure 10 with the plugging material slurry 8 injected were not lifted at the least. If the bottom plate of the container 40 were slid after the ceramic honeycomb structure 10 is lifted, negative pressure would be applied to the plugging material slurry 8 forming the plugs 4 while opening the bottom of the container 40, so that the plugging material slurry 8 undergoes sudden pressure rise from reduced pressure to atmospheric pressure, making it likely to generate the recesses 7 at the ends 45 of the plugs 4.
JP 2004-290766 A discloses a method comprising pressing a slurry 8 in a container (not shown) into flow paths 3; and removing a slurry 8a attached to the end surface 5 of the ceramic honeycomb structure 10 with a spatula 43 while separating the ceramic honeycomb structure 10 from the container 40 by sliding a bottom plate 42 of the container 40, as shown in FIG. 6(c). This reference discloses that the forced separation of the slurry 8 inside the flow paths 3 from that outside the flow paths 3 can prevent the generation of recesses due to the negative pressure and the viscosity of the slurry. However, because the slurry 8a attached to the end surface of the honeycomb structure is removed before solidified in the method of JP 2004-290766 A, the recesses 7 may be generated in the course of drying the slurry 8.